Crystallization apparatus and method

ABSTRACT

A crystallization apparatus and method wherein both the size and amount of crystal particles withdrawn through a fines removal circuit from a slurry body, at least a portion of which is maintained under supersaturation conditions, are independently regulated to provide product crystals of substantially improved size uniformity.

United States Patent [1 1 [111 3,873,275 Bennett [4 Mar. 25, 1975 CRYSTALLIZATION APPARATUS AND 3,416,889 12/1968 Caldwell 23/273 THO 3,459,509 8/1969 Aoyama 1 23/273 3,530,924 9/1970 Domning 23/273 Inventor: Richard tt, P orest, 111. 3,556,733 1/1971 Hedrick 23/273 [73] Assignee: Whiting Corporation, Harvey, 111. OTHER PUBLICATIONS [22] Filed: Apr. 21, 1972 Newman and Bennett, Circulating Magma Crystalliz- 1 pp NO: 246,439 ers, Chem. Eng. Prog., Vol. 55, pp.65-70 (1959).

Related U.S. Application Data Primary liraminer-A Louis Monacell [63] Continuation of Ser. No. 861,805, Sept. 29, 1969, ASA/W"! Exam/"EH13- Slmders abandoned, Attorney, Agent, or FirmLockwo0d, Dewey, Zickert & Alex [52] U.S. Cl. 23/273 R, 23/301 R [51] int. Cl B0ld 9/02 [57] ABSTRACT [58] Fleld of Search 23/273 R, 301 R A Crystallization apparatus and method wherein both the size and amount of crystal particles withdrawn [56] References cued through a fines removal circuit from a slurry body, at

UNITED STATES PATENTS least a portion of which is maintained under supersatl,945,281 1/1934 Leithauser 23/273 uration conditions, are independently regulated to 2,856,270 10/1958 Saeman 23/273 provide product crystals of substantially improved size 3,071,447 1/1963 Bernhardi 23/273 uniformity 3,383,180 5/1968 Kralik et al. 23/273 14 Claims, 16 Drawing Figures FEE .n. "*-...,,M 52 PRODUCT n Pf?! 'TijU HAR 2 5 i975 SHEET 2 or 4 INVENTOR RICHARD C. BENNETT CRYSTALLIZATION APPARATUS AND METHOD This is a continuation of application Ser. No. 861,805, filed Sept. 29, 1969, now abandoned.

BACKGROUND AND DESCRIPTION OF THE INVENTION The present invention generally relates to improve ments and innovations for providing improved equilibrium conditions within a crystallization vessel. More particularly, this invention is concerned with an improved crystallization apparatus and method wherein both the size and quantity of crystal fines removed from a slurry body undergoing crystallization are regulated.

Various desired objectives for improving product size and uniformity in crystallization processes are known in the art. These include: reducing the level of supersaturation to correspond to low rates of nuclei formation; maintaining adequate seed crystals both as to size and quantity upon which the deposition of the generated supersaturation may take place; contacting the seed with the supersaturated liquid as soon as possible in order to avoid losses due to time decay; minimizing mechanical stimulus and attrition which promotes unwanted nucleation; providing insofar as possible, conditions of temperature, viscosity, interfacial tension, surface tension and impurities which favor the growth of the desired product; and, removing excess nuclei or fine crystals as soon as possible after their formation.

The present invention provides an improved crystallization apparatus and method by which cycling in the size distribution of product crystals can be substantially avoided. In accordance with the present invention the mother liquor recirculation rate and the size of crystal particles removed through a fines destruction circuit are independently regulated. In this manner, not only are undesirable fines removed from the slurry body undergoing crystallization, but also, the residence times of such fines is effectively regulated to provide a product of substantially improved size uniformity.

It is, therefore, an important object of the present invention to provide a crystallization apparatus and method wherein optimum equilibrium conditions are achieved for producing a crystal product of uniform size over a given period of time.

Another object of the present invention is to provide a new and improved crystallization apparatus and method wherein the size and quantity of fine crystals removed from a slurry body through a fines removal circuit are independently regulated.

Another object of the present invention is to provide a new and improved crystallization apparatus and method in which the residence time of crystal particles below a predetermined size is effectively controlled to achieve optimum equilibrium conditions.

Other objects and advantages of the present invention will be apparent from the accompanying detailed description thereof taken in conjunction with the drawings wherein:

FIG. 1 is a vertical sectional schematic view of a crystallization apparatus embodying the present invention;

FIG. 2 is a plan sectional view taken along the line 2-2 of FIG. 1;

FIG. 3 is a vertical sectional schematic view of a modified crystallization apparatus embodying principles of the present invention;

FIG. 4 is a plan sectional view taken along the line 44 of FIG. 3;

FIG. 5 is a vertical sectional schematic view of another modified crystallization apparatus embodying the present in vention;

FIG. 6 is a plan sectionalview taken along the line 6-6 of FIG. 5;

FIG. 7 is a vertical sectional schematic view of another modified crystallization apparatus embodying the present invention;

FIG. 8 is a plan sectional view taken along the line 88 of FIG. 7;

FIG. 9 is a vertical sectional schematic view of another modified crystallization apparatus embodying the present invention;

FIG, 10 is a plan sectional view taken along the line l0-l0 of FIG. 9;

FIG. 11 is a vertical sectional schematic view of another modified crystallization apparatus embodying the present invention; and

FIG. 12 is a plan sectional view taken along the line 12-12 of FIG. 11.

FIG. 13 is a vertical sectional schematic view of another modified crystallization apparatus embodying the present invention;

FIG. 14 is a plan sectional view taken along the line l414 of Flg. 13;

FIG. 15 is a vertical sectional schematic view of another modified crystallization apparatus embodying the present invention; and

FIG. 16 is a plan sectional view taken along the line l6l6 of FIG. 15.

Referring to the drawings and with particular reference to FIGS. 1 and 2, the numeral 11 generally designates an improved crystallization apparatus constructed in accordance with the present invention which includes a main tank or vessel 12 having a tapered bottom section 13 that extends into a fixed diam eter mid-section l4 and frusto-conical top section 15 joined to a generally cylindrical column 16. As is shown, column 16 includes an upper side wall portion 17a extending above the main vessel 12 and a lower side wall portion 17b which defines an exterior baffle within the main vessel 12. The upper side wall portion 17a of column 16 is terminated in a top dome section 18 and is provided with a suitable vapor outlet 19 which communicates the interior freeboard space 21 above the slurry level 22 with a suitable vacuum source (not illustrated) for maintaining supersaturation conditions within the slurry body portion adjacent slurry level 22.

In the embodiment illustrated in FIGS. 1 and 2 a draft tube 23 is suitably supported in known manner by a plurality of spokes 24 which radially extend coaxially within the vessel 12 and column 16. Circulation of the slurry is provided by apropeller 25. Propeller 25 is suitably supported by a shaft which in turn is driven by a suitable drive means (not illustrated), such as, for example, an electric motor. The propeller serves to circulate the liquid slurry within the vessel 12 and column 16 in a predetermined flow path and is operated at a sufficient fluid velocity so as to suspend all solid particles within the slurry. In the illustrated embodiment, the predetermined flow path is in the form of a torroidal pattern which passes upwardly through the draft tube 23 and then downwardly along the exterior thereof. If desired, the action of propeller 25 can be reversed so that the direction of the flow in the torroidal flow pattern is likewise reversed.

As previously noted, the lower side wall portion 17b of column 16 provides a generally cylindrical baffle which, in cooperation with the cylindrical side wall portion 14 of vessel 12, defines a column of substantially uniform annular cross-section from bottom to top. The lower end of this uniform cross sectional area is adjacent the torroidal flow path within the vessel 12 and 16, however, in accordance with the present invention, the zone defined by baffle section 17b and side wall 14 is generally quiescent with respect to said predetermined flow created by the propeller 25.

As is best shown in FIG. 2, the annular cross sectional area surrounding baffle 17b is divided into at least two separate elutriation chambers by means of a plurality of vertically extending partitions 27 and 28. In the illustrated embodiment of the invention, partitions 27 and 28 extend from the bottom end 17c of baffle 17b up to the upper portion of inwardly tapered top portion of the vessel 12 to completely divide the cross sectional area into separate elutriation chambers 29 and'3l. It will be appreciated, however, that while the embodiment illustrated in FIG. 1 discloses two separate elutriation chambers, with chamber 29 being approximately twice the volume of chamber 31, the present invention is not limited to this particular two chamber arrangement but may include any division of the elutriation column into a plurality of elutriation chambers so long as at least two separate elutriation columns are provided. In this regard, however, it should further be noted that the particular arrangement illustrated in the embodiment shown in FIGS. 1 and 2 is preferred in that it enables operation of the crystallization apparatus 11 with any of three elutriation chamber volumes, viz. one times that contained in chamber 31 (by using chamber 31 only), two times that contained in chamber 31 (by using chamber 29 only) and three times that contained in column 31 (by using both chambers 29 and 31).

In accordance with a further aspect of the present invention, each of the elutriation chambers 29 and 31 is provided with a fines removal outlet connection 32 and 33 respectively. As is shown, fines removal outlet connection 32 communicates with recycle lines 34a, 34b and 34c. Suitable valve means 35 can be provided to controlthe flow of slurry and fine particles from fines removal outlet 32. Likewise, the fines removal outlet 33 communicates with fines removal flow lines 36a, 36b and 360. Valve means 37 is provided for controlling the flow of liquid through the fines removal outlet 33.

In the illustrated embodiment, each of the fines re-' moval process lines 34a-c and 360-0 communicate with a single fines destruction circuit 40 through a tee member 38 having a process line 39 which communicates the fines removal lines 340 and 360 with the inlet 41a of a heat exchanger 41 wherein the crystal particles contained in said fines removal lines are solubilized. If desired, separate fines destruction circuits may be provided for each of the fines removal outlets 32 and 33.

Likewise, other solubilizing means may be used in place of, or in conjunction with, heat exchanger 41. Such other means can include, for example, suitable devices for the addition of a solvent (such as water) to the flow either in the fines destruction circuit 37 or at some other suitable location such as, for example, in the elutriation chambers 29 and 31 adjacent fines removal outlets 32 and 33, respectively.

Outlet 41b of heat exchanger 41 discharges into the suction side of a variable delivery pump 42 whichreturns the flow of dissolved crystal fines and recycle liquid slurry to vessel inlet 43 via flow lines 44ae. If desired, dissolved crystals and liquid slurry can be discharged to storage or other processing the plant via discharge line 45. In this regard, valves 46 and 47 can be selectively opened or closedin accordance with the desired flow path for the dissolved solids and liquid slurry.

In the embodiment shown in FIGS. 1 and 2, the flow from recycle lines 34a-c and/or 36a-c can be selectively discharged to storage or further processing remote from crystallization apparatus 11 via discharge lines 4811 and 48b which, as illustrated, are connected to a variable flow pump 49 which in turn discharges to such storage or further processing as is desired for the system. In this regard, valve means 51, 52 and 53 are provided to effect the desired flow through the fines destruction circuit 37 or to discharge lines 48a and b.

In operation, the vessel 12 is preferably maintained at the level indicated at 22 with a slurry which includes crystal particles and liquid. Evaporation ofliquid of solution is accomplished by maintaining vacuum conditions in the freeboard space 21 in order to maintain supersaturation conditions in at least a portion of the slurry body.

A closed flow circuit is created and maintained by propeller 25 which operates conjointly with draft tube 23 to direct the flow in a generally torroidal pattern as is generally indicated by the solid arrows in FIG. 1. As is shown, the liquid flows upwardly through the draft tube 23 and then spreads outwardly over the top of the draft tube and downwardly along the outside of the draft tube until it contacts the generally tapered section 13 adjacent the bottom of the vessel from which it is deflected and flows upwardly through the draft tube 23. Propeller 25 is driven at a rate which will be sufficient to suspend the largest crystals or particles but which also does not destroy the generally quiescent zone within the elutriation columns 29 and 31. Under these conditions, the lower ends of elutriation chambers 29 and,31 are in open communication with the outer portion of the closed flow circuit. Feed slurry is fed through a feed inlet 54 which, if desired, can be in communication with recycle return line 44d. Likewise, coarse product crystals are removed on a steady state basis through product outlet 55, positioned in the tapered bottom portion 13 of vessel 12. Feed material is fed into the vessel at a rate which will maintain the operating level of the liquid substantially as shown at 22 and compensate for materials withdrawn through the fines removal outlets 32 and 33, product outlet 55 and vapor inlet 19.

Since the lower end of the elutriation chambers 29 and 31 are on the fringe or periphery of the main flow circuit, the main flow which is created by propeller 25 does not operate to carry any of the larger particles or crystals into these elutriation chambers. Flow through the elutriation chambers 29 and/or 31 can be controlled through the variable flow pump 42 in the fines destruction circuit or variable flow pump 49 in the alternative discharge circuit. In accordance with one as pect of this invention, the flow velocity is controlled to provide a flow rate through the respective fines removal outlets 32 and/or 33 which will operate to suspend and remove only particles ofa predetermined size or smaller. Various equivalent arrangements for controlling flow velocity in the elutriation chambers 29 and 31 will be apparent to those skilled in this art. These include, for example, the use of individual variable flow pumps in each of the flow lines 34a-c and 36a-c or, the use of a constant flow pump with a flow control valve.

In accordance with an important aspect of the present invention, the residence time of crystal fines in the vessel 12 is controlled so that such residence time is less than the residence time which corresponds to the size of particles to be removed through the fines removal outlets. For example, in crystallization apparatus 12 by closing valves 35, 53 and 46, and opening valves 37, 51 and 47, crystal fines are removedfrom fines removal outlet 33 only. The size of the such fine particles to be removed is determined by the velocity in elutriation chamber 31 which in turn is controlled by variable flow discharge pump 42. If a greater volume of mother liquor is desired to be removed from the slurry body without varying the size of the crystal fines removed, fines removal outlet 32 its associated elutriation chamber 29 can be used alone or in conjunction with outlet 33 and chamber 31. For example approximately twice the volume of mother liquor removed from outlet 31 can be removed from the slurry body by selectively operating apparatus 11 with valves 35, 52, 51 and 47 in the open position and valves 37 and 46 closed. The fines contained in elutriation chamber 29 will thus be removed from such chamber through the fines removal outlet 32, solubilized by heat exchanger 41 and then returned to the main slurry body return inlet 43. If it is desired to triple the volume of mother liquor removed, both chambers 29 and 31 can be used with valves 35, 37, 52, 51 and 47 in the open position and with valves 53 and 46 closed. In each case, the velocity in the respective chambers can be independently regulated (with respect to the volume of mother liquor removed) to provide controlled removal of crystal fines below a predetermined size only.

Referring to FIGS. 3 and 4, a further embodiment of the crystallization apparatus of the present invention is generally designated by the reference numeral 61. As is shown, apparatus 61 includes a vessel 62 having a generally cylindrical fixed diameter section 63, which at its upper end, extends into an inwardly tapered top section 64 and which, at its lower end, tapers into a generally conical bottom section 65. A full flow recycle outlet 46 is provided in bottom vessel section 65 which communicates with the suction side 68a of centrifugal pump 68 through a recycle process line 67. The discharge side 6812 of centrifugal pump 68 communicateswith recycle inlet 69 formed in the bottom section 65 of vessel 62. If desired, a heat exchanger 71 may be provided for supplying additional heat, where necessary, to achieve the desired supersaturation conditions in the slurry body. Typically, such a heat exchanger would be employed where feed materials are relatively dilute.

As is shown in FIG. 3, top section 64 of the vessel 62 extends into a vertically extending column haing a side wall 73a extending above vessel 62 and side wall portion 73b which extends internally of vessel 62 and which, together with the fixed diameter section 63 of vessel 62, defines an annular column of substantially uniform cross sectional area. Column 72 can be closed off at its upper end by a domed portion 74 and can be provided with a suitable vapor outlet 75 which commu nicates with a suitable vacuum source (not illustrated).

municates with an outwardly tapered and upwardly extending feed and recycle flow line 77 having an open upper end 77a through which feed and recycle flow from pump 68 is discharged to provide a predetermined flow pattern substantially as depicted by the arrows.

In accordance with an important aspect of the present invention and as is best shown in FIG. 4, the annular area between baffle 73b and fixed diameter wall 63 is divided into a plurality of separate compartments or chambers 78, 79, 81 and 82 by baffles 83, 84, and 86. These sub-compartments are respectively provided with fines removal outlets 78a. 79a, 81a and 82a. As shown in FIG. 3, fines removal outlet 79a communicates with the suction side of a variable flow pump 87 through associated fines removal process piping 79b, 79c and 790' which in turn connects to one inlet in a suitable tee member 88 and through said tee member 88 into the fines removal process lines 89a and 8911 which in turn are connected to the suction side of variable flow pump 87. Fines removal outlet 82a similarly communicates with the suction side of pump 87 through associated piping 82b, 82c, and 82d and process lines 89a and 89b. While not specifically shown in the drawings, it will be appreciated that the fines removal outlets 78a and 81a, in like manner, communicate with common fines process lines 89a and 89b. The discharge side of variable discharge pump 87 can com municate with a fines destruction circuit similar to fines destruction circuit 37 described in detail in connection with the embodiment shown in FIGS. 1 and 2. Such fines destruction circuit includes fine particle crystal solubilizing means which, in accordance with one aspect of this invention, is used to solubilize the crystal fines withdrawn from the elutriation chambers prior to the return thereof to the mother liquor return line 91.

In accordance with an important aspect of the present invention, each of fines removal process lines 79c and 82c is equipped with a suitable valve 7% and 82a respectively which can be selectively opened to control the flow of mother liquor through each of the respective fines removal outlets.

Similarly, while not illustrated, the process lines associated with fines removal outlets 78a and 81a are equipped with flow control valves to control the flow of mother liquor therethrough. The operation of crystallization apparatus 61 will be apparent from the previous description of the operation of crystallization apparatus 11 (FIGS. 1 and 2). Accordingly, a detailed description thereof is not necessary. In this regard, however, it will be noted that complete or partial flow in accordance with a fixed amount of suction from pump 87 can be effected in each of the elutriation chambers by opening or closing the respective valve 78e (not illus trated), 79a, 81e (not illustrated) and 82e to the extent desired. In this manner, the size of crystal particles removed from each of the fines removal outlets can be individually controlled along with the volume of mother liquor (the quantity of crystal particles below a given predetermined size) to achieve crystallization conditions within the slurry contained in the vessel for improving crystal product size and uniformity in the product size.

In the embodiment shown in FIGS. 5 and 6, a further modified form of the present invention is shown and generally designated by the reference numeral 101. As

is illustrated, crystallization apparatus 101 includes a generally cylindrical vessel 102 having a fixed diameter mid section 103 the upper end of which extends into an inwardly tapered top portion 104 and the bottom of which extends into a tapered conical section 105 having a large diameter full flow recycle outlet 106 formed therein. A suitable swirlbreaker such as is generally indicated by the reference numeral 107 can be provided in the lower portion of the conical section 105. Recycle outlet 106 communicates with the suction side 108a of a centrifugal pump 108 through a recycle flow line 109. Likewise, the discharge side 108b of pump 108 communicates with a recycle inlet 111 through a recycle line 112. If desired, heat exchanger 113 can be provided in recycle line 112 where the use of additional heat is required for the particular crystallization operation being conducted in the apparatus 101.

Upper tapered portion 104 of the generally cylindrical vessel 102 communicates with a column 114 having a tapered top end wall 115 and generally fixed diameter section 116a which is provided with a vapor outlet 117 in direct communication with a suitable vacuum source (not illustrated) for maintaining subatmospheric conditions in the freeboard space 118 above the slurry liquid level 119 therein. Upper side wall portion 116a extends into a baffle defining portion 116b which, together with side wall 103 of vessel 102 generally defines a uniform cross sectional annular area therewith.

In accordance with an important aspect of the present invention, the annular area between baffle 1161) and straight wall section 103 is divided into a plurality of separate compartments by means of vertically extending partitions 123 and 124.

As is best shown in FIG. 5, elutriation column 122 includes a fines removal outlet 122a adjacent the upper end thereof which communicates with associated mother liquor removal process lines 122bd. Similarly, fines removal outlet 121a communicates the elutriation column 121 with mother liquor flow lines 121b-d. Each of mother liquor lines 12lb and 121d communicated through a coupling 123 to a common flow line 124 which in turn is connected to the suction side of a variable flow pump 125. Flow control valves 121e and 1222 can be selectively opened or closed to control the flow rate ofmother liquor removed via fines removal outlets 121a and 122a, respectively. The discharge side of variable flow pump 125 communicates with a suitable fines destruction circuit (not illustrated) such as designated by the reference numeral 37 in the embodiment of the present invention described in connection with FIGS. 1 and 2. Mother liquor which has been processed in the fines destruction circuit is returned to recycle flow line 109 at inlet 126.

As is best shown in FIG. 6, elutriation column 121 is approximately one-half the size of elutriation column 122. In this manner, separate control over the size of crystal particle fines removed via the elutriation columns can be controlled independently of the volume of mother liquor removed. As such, the residence time of crystal particles below a predetermined size in the crystallization apparatus 101 can be effectively controlled to provide a uniform product size crystal.

In the embodiment of the present invention shown in FIGS. 7 and 8, a modified form of a crystallizer embodying principles of the present invention is generally shown and designated by the reference numeral 151. As is shown, crystallization apparatus 151 includes a generally cylindrical vessel 152 of substantially fixed diameter having an enclosed top portion 153. A vapor outlet 154 communicates the freeboard space 155 above the slurry liquid level 156 in the vessel with a suitable vacuum source (not illustrated). Main body section 152 of the crystallizer extends adjacent its lower end into a tapered bottom section 157 having a centrally disposed full flow recycle outlet 158 which communicates with the suction side 159a of a pump 159 through a suitable recycle flow line 161. The discharge side 15912 communicates with a recycle inlet 162 formed in the apparatus 151 through a recycle flow line 163. If desired, additional heat can be provided to the recycling slurry through a suitable heater 164. Recycle inlet 162 communicates with interior recycle flow distribution process lines 165 and 166. As is shown, recycle line 166 extends into a U-shaped top portion 167 and downwardly extending side wall portion 168 which, together with recycle flow line 166, defines a generally annular area of uniform cross section around the exterior surface of recycle feed distribution line 166.

As is best shown in FIG. 8, the annular area around recycle flow line 166 is divided into a pair of separate elutriation chambers 169 and 171 both of which are open adjacent their lower end and of substantially uniform cross section for at least a substantial portion of their height. In accordance with one aspect of the present invention, elutriation chambers 169 and 171 constitute a quiescent zone in the vessel 151 since the flow path of recycle slurry developed by the pump 159 is substantially around the exterior walls 168 only.

Each of elutriation chambers 169 and 171 is provided with fines removal outlets 169a and 171a, respectively. Fines removal outlet 169a communicates with a coupling member 172 through a plurality of mother liquor recycle flow lines 169b-d. Likewise, mother liquor recycle outlet 171a communicates with the coupling member 172 through mother liquor recycle flow lines 171b-d. Each of these mother liquor recycle flow lines communicates with the suction side of a variable flow pump 173 through a common mother liquor recycle flow line 174. Variable flow pump 173 in turn communicates with a suitable fines destruction circuit such as was generally defined by the reference numeral 40 in connection with the embodiment shown and described in FIGS. 1 and 2. Mother liquor in which the undesirable fine crystal particles have been destroyed are returned to the system through a suitable return line 175. Control of the flow through the fines removal outlets 169a and 171a is effectively accomplished through valves 169e and 171e, respectively. In this manner, and as was previously noted, separate control of the quantity of fines as well as the size of fines is provided to enable the production of a uniformly sized product crystal.

FIGS. 9 and 10 illustrate a crystallization apparatus 181 which is similar in general construction to crystallization apparatus 11 of FIGS. 1 and 2. As is shown, ves sel 181 includes a fixed diameter mid-section 182 which extends at its upper end into a frusto-conical section 183 joined to a generally cylindrical column 184 having an upper side wall portion 184a and a lower side wall portion l84b. Upper side wall portion 184a is closed off by a dome 184C and provided with an outlet 184a which communicates the freeboard space 185 with a suitable vacuum source (not illustrated). The

lower end of vessel mid-section 182 extends into a rounded or domed bottom 186.

A draft tube 187 is mounted to column Wall 1841? by a plurality of spokes 188. Forced circulation of the slurry is provided by an impeller 189 connected to a shaft 190 and driven by a suitable drive means (not illustrated). Product crystals are withdrawn from product outlet 191 and slurry feed can be introduced into the vessel at 192.

The principle difference between the crystallization apparatus 181 and that shown in FIGS. 1 and 2 is the construction of the concentric elutriation chambers 193, 194 and 195. As is shown, chamber 193 is defined by vessel section 182 and interior wall 196. Likewise, chamber 194 is defined by interior walls 196 and 197. Chamber 195 is similarly defined by interior wall 197 and column wall 184b.

Each of the chambers 193, 194 and 195 are open at their lower ends and in communication with the main slurry body contained in the vessel 181 adjacent the periphery of the flow path developed by impeller 189. Likewise, each of chambers 193, 194 and 195 is of uniform crosssection for a substantial portion of the height thereof. Fines removal outlets 193a, 194a and 195a, respectively, communicate chambers 193, 194 and 195 with associated flow pipes 193b, 1941) and 195b each of which is equipped with flow control valves 1930, 1940 and 1956. Pines removal lines 198a and 198b likewise connect each of the flow pipes 193b, 194b and l95b with a suitable solubilizing means which, in the illustrated embodiment, is a heater 199. A variable flow pump 200 communicates on its suction side to heater 199 and on its discharge side to the mother liquor return inlet 201. If desired, an auxiliary discharge line 202 and valve 203 can be provided for selective discharge of the mother liquor to storage or other desired processing location in the plants.

FIGS. 11 and 12 illustrate the application of the present invention to a forced circulation crystallizer which is generally designated by the reference numeral 210. As is shown, crystallizer 210 includes an evaporating chamber 211 having a recycle flow line 212 which communicates evaporating chamber 211 with the suction side of a centrifugal pump 213. The discharge side of pump 212 communicates with evaporating chamber 211 through a return line 214 which, if desired, can be provided with a suitable heater 215. As is shown in FIG. 11, evaporating chamber 211 includes an overhead vapor outlet 2l1a which communicates with a suitable vacuum source (not illustrated).

In accordance with the present invention, a portion of the flow through process line 214 is withdrawn through fines circuit process feed lines 2l6a-.d and introduced into a fines separation vessel 217. As best shown in FIG. 12, vessel 217 is divided by a plate 218 into two separate elutriation chambers 219 and 220 each of which is respectively provided with a fines removal outlet 219a and 220a adjacent the upper ends thereof. Process lines 2191; and 220b, respectively, communicate fines removal outlet 219a and 220a with a suitable fines destruction circuit (not illustrated) such as that previously described in connection with the embodiment shown in FIGS. 1 and 2. In accordance with the present invention and as is more fully described in connection with the previous embodiments, flow control valves 219C and 220C enable independent control of both the volume and size of crystal particle fines removed from the vessel 217. Recycle flow of crystal particles and slurry liquid which are not removed via the fines removal outlets 219a and 220:: are returned to recycle line 212 through the return line 22111-0. As shown, the return flow of dissolved crystal fines and recycle liquid slurry from the fines destruction circuit can be introduced into the return line 2211).

FIGS. 13 and 14 illustrate the application of the present invention to an internal baffle design crystallizer 230. As is shown, crystallizer 230 generally includes a main vessel 231 having a generally fixed diameter midsection 232 which is closed at its upper end by a domed section 233 and at its lower end by a generally frustoconical bottom section 234. A suitable vapor outlet 235 which communicates with a vacuum source (not illustrated) is provided for maintaining supersaturation conditions within the slurry body contained in the vessel 231.

As is shown in FIGS-1 and 2, vessel 231 includes a draft tube 236 which is supported in a known manner by a plurality of spokes 237. Circulation of the slurry in the vessel is provided by means of a suitable propeller 238 which is supported by a shaft 239 and driven by a suitable drive means (not illustrated), such as, for example, an electric motor. The propeller serves to circulate the entire mass of slurry within the vessel 231 in a predetermined flow path and is operated at a sufficient fluid velocity so as to suspend all solid particles within the slurry. In the illustrated embodiment, this predetermined flow path is in the form of a torroidal pattern which passes upwardly through the interior of the draft tube 236 and then downwardly along the external thereof. If desired, propeller 238 can be reversed to ef fect a correspondingly reverse torroidal flow pattern.

As best shown in FIG. 13, vessel 231 is equipped with an internal baffle 240 which includes an upper frustoconical portion 240 the upper end of which sealingly engages with the interior wall of fixed diameter section 232. The lower end of frusto-conical internal baffle section 230a extends into a fixed diameter baffle section 24% which, in cooperation with the cylindrical side wall 232 of vessel 231, defines a column of substantially uniform annular cross-section. The lower end of this uniform cross-sectional area is adjacent the torroidal flow pattern within the vessel 231. In accordance with one aspect of the present invention, however, the zone defined by baffle section 24% and side wall 232 is generally quiescent with respect to the predetermined flow created by the propeller 238.

The annular area between fixed diameter baffle sec tion 24% and cylindrical wall 232 is, in accordance with an important aspect of the present invention, divided into a plurality of separate compartments by means of vertically extending partitions. In the illustrated embodiment, partitions 241 and 242 cooperate to define two separate elutriation columns 243 and 244 which communicate with fines removal outlets 243a and 244a, respectively. As shown, fines removal outlet 243a communicates with a suitable heater 245 or equivalent fines destruction device through associated mother liquor removal process lines 243b-f. Similarly. fines removal outlet 244a communicates with a suitable heater of the equivalent fines destruction device 246 through its associated mother liquor removal process lines 244b-f. Flow control valves 243g and 244g are provided and can be selectively opened or closed to control the flow of mother liquor removed through fines removal outlets 243a and 244a, respectively.

The outlet of the heater 245 discharges into the suction side of a variable delivery pump 247 which returns the flow of dissolved crystal fines and recycle liquid slurry to the interior of vessel 231 through flow lines 248a-d. If desired, such dissolved crystals and recycle liquid slurry can be selectively discharged to storage or other processing through discharge line 249 by opening control valves 250 and closing control valve 251.

Similarly, the discharge side of heater 246 communicates with a variable delivery pump 252 which returns the flow of dissolved crystal fines and recycle liquid slurry to the interior of vessel 231 through flow lines 253ad. As is apparent from FIG. 13, dissolved crystals and liquid slurry likewise can be selectively discharged to storage facilities or other processing through discharge line 254 by selectively opening valves 255 and closing valve 256.

In accordance with one aspect of the present invention, elutriation column 243 is approximately one half the size of elutriation column 144. In this manner, separate control over the size of crystal particle fines removed through the elutriation columns can be provided independently on the volume of mother liquor removed. As such, the residence time of crystal particles below a predetermined size in the crystallization apparatus 231 can be effectively controlled to provide a uniform product size crystal.

FIGS. 15 and I6 illustrate the application of the present invention to a forced circulation surface cooled crystallizer which is generally designated by the reference numeral 265 and which includes a retention vessel 266, a circulating pump 267 and a heat exchanger 268. As is best shown in FIG. 15, recycle slurry contained in the retention vessel 266 communicates with the suction side 267a of the circulating pump 267 through a recycle flow line 269 and is discharged from the pump to a process inlet 271 in the heat exchanger 268 through a flow line 270. The process outlet side 272 of heat exchanger 268 communicates with the retention vessel process inlet 273 through a flow line 274. A suitable coolant is circulated through the shell side of the heat exchanger 268 by means of the shell side inlet and outlet connections 275 and 276, respectively. In this manner, supersaturation is produced in the slurry being circulated through the heat exchanger 268.

Short circuiting of the slurry flow through the retention vessel is prevented in the illustrated embodiment by means of an inlet extension pipe 277 which directly receives the return recycle flow from the recycle inlet 273 and distributes such flow adjacent the surface of the slurry contained in the vessel 266 to produce a flow pattern within the vessel 266 corresponding to that generally indicated by the arrows.

In accordance with the present invention, vessel 266 includes an interior baffle 278 which, in cooperation with the cylindrical side wall section 266a of vessel 266, defines a column of substantially uniform annular cross-section. The zone defined by the baffle section 278 and side wall 266a is generally quiescent with respect to the flow in the main portion of the vessel 266.

In accordance with an important aspect of the present invention and as is best shown in FIG. 16, the annular area between baffle 278 and fixed diameter wall section 266a is divided into a plurality of separate elutriation columns by means of vertically extending partitions. In particular, in the illustrated embodiment, partitions 279-282 cooperate to define four separate elutriation columns 283-286. Each of the elutriation columns 283-286 is provided with an individual fines removal outlet 283a-286a, respectively, which communicates with an associated fines destruction circuit. For example, fines removal outlet 283a communicates with a suitable heater or equivalent fines destruction device (not illustrated) through its associated fines removal process lines 2831; and c. Flow control valve 287 is provided in the process line section 28312 and can be selectively opened or closed to control the flow of mother liquor removed through the fines removal outlet 2830. Likewise, fines removal outlet 285 a communicates with the fines destruction circuit through a process line 285b which is equipped with a similar flow control valve 288. While-not specifically shown, it will be appreciated that flow lines connect fines removal outlets 284a and 286a with the fines destruction circuit and are each provided with separate flow control valves. In this manner, separate control over the size of crystal particle fines removed through the elutriation columns can be provided independently of the volume of mother liquor removed. Accordingly, the residence time of crystal particles below a predetermined size in the crystallizer apparatus 265 can be effectively controlled, in accordance with the present invention, to provide a uniform product size crystal. In the illustrated embodiment, the return flow of crystal particle fines and liquid slurry can be introduced into the recycling process stream at the mother liquor return 289 in flow line 269.

It will, of course, be apparent that in each of the above described embodiments complete fines destruction is not essential to the practice of the present invention. Since the smallest crystals are the first to dissolve and since such smallest crystals generally represent the most harmful portion in a slurry body, destruction of such portion may be sufficient to effect the desired degree of product size control.

While in the foregoing specification certain embodiments have been described in detail, it will be appreciated that modifications and variations to the heretofore described embodiments will be apparent to those skilled in this art. Accordingly, the present invention is to be limited only by the scope of the appended claims.

I claim:

1. An improved crystallization apparatus, comprising a vessel adapted to contain a body of slurry which includes crystal particles and liquid, means for maintaining supersaturation conditions in the slurry body to induce crystallization therein, means in said crystallization apparatus for circulating said slurry in a predetermined flow path in said vessel and for maintaining said crystal particles in suspension, said vessel also including at least two separate elutriation chambers, each of said elutriation chambers having an outlet at one end thereof for removing slurry liquid and crystal particles which are below a predetermined size only from said slurry body, and means communicating with each of said outlets for independently regulating the flow rate in each of said elutriation chambers to selectivelyopen ate any one or more of said elutriation chambers at a desired flow rate; whereby, the size and quantity of crystal particles removed from said slurry body through said elutriation chambers can be selectively and IIltlC' pendently regulated with respect to each other to provide product crystals of improved size uniformity.

2. The improved crystallization apparatus of claim 1 wherein each of said elutriation chambers in said apparatus is of substantially uniform cross-section for at least a substantial portion of its height.

3. An improved crystallization apparatus, comprising a vessel adapted to contain a body of slurry which includes crystal particles and liquid, means in said vessel for maintaining a supersaturation zone in said slurry body to induce crystallization therein, means in said vessel for circulating a substantial portion of said slurry body in a predetermined flow path through said supersaturation zone and for maintaining said crystal particles in suspension in said liquid, a plurality of elutria tion chambers in said vessel, each of said elutriation chambers having a substantially uniform crosssection for at least a substantial portion of its height and a lower end which is in open communication with said substantial portion of said slurry body adjacent the predetermined flow path in a generally quiescent zone with respect thereto, each of said elutriation chambers also including an outlet adjacent the upper end thereof, means communicating with each of said outlets for independently regulating the flow rate in each of said elutriation chambers to withdraw a recycle stream of crystal particles and slurry liquid from said zone and to selectively operate any one or more of said elutriation chambers at a desired flow rate, thereby enabling the size and quantity of crystal particles removed from said slurry body to said recycle stream to be selectively and independently regulated with respect to each other, and means for removing crystal particles in said recycle stream prior to the return of said stream to the body of slurry in said vessel.

4. An improved crystallization apparatus, comprising a vessel adapted to contain a body of slurry which includes crystal particles and liquid, first means for maintaining supersaturation conditions in the slurry body to induce crystallization therein, second means in said crystallization apparatus for circulating said slurry in a predetermined flow path in said vessel and for maintaining said crystal particles in suspension, a plurality of elutriation chambers in said apparatus, each of said elutriation chambers having a substantially uniform cross-section for at least a substantial portion of its height and having an inlet at one end thereof and an outlet at the opposite end thereof, said inlet communicating with said body of slurry at a location therein whereby slurry will be received in said elutriation chambermeans from a generally quiescent zone with respect to said predetermined flow path, means communicating with each of said elutriation chambers for independently regulating the flow rate in each of said elutriation chambers to selectively operate any one or more of said elutriation chambers at a desired flow rate; whereby, both the size and quantity of crystal particles in said slurry stream removed from said slurry body can be selectively and independently varied with respect to each other to provide product crystals of improved size uniformity.

5. The improved crystallization apparatus of claim 4 wherein said vessel includes two separate elutriation chambers, the first of which has a cross-sectional area which is equal to approximately one half the crosssectional area of said second elutriation chamber.

6. An improved crystallization apparatus, comprising an enclosed vessel adapted to contain a slurry body of liquid and crystal particles, vacuum means communicating with said vessel for maintaining a supersaturation zone in said slurry body, forced circulation means in said vessel for circulating a substantial portion of said slurry body in a predetermined flow path through said supersaturation zone and for maintaining said crystal particles in suspension, said vessel including a mid-section defined by a generally cylindrical shell and an interior wall co-axially disposed with re spect to said cylindrical shell, said interior wall and cylindrical shell defining a channel having a uniform an nular cross-section for at least a substantial portion of its height, and partition members radially disposed between said interior wall and cylindrical shell defining said channel into separate elutriation chambers, each of said elutriation chambers having an inlet adjacent the lower end thereof communicating with said substantial portion of the slurry body adjacent the periphery of said predetermined flow path and having an outlet above said inlet for withdrawing a recycle stream of slurry liquid and crystal particles, variable flow rate means communicating with each of said outlets for withdrawing said recycle stream therefrom at a rate which can be selectively controlled to establish a flow velocity in the elutriation chamber associated with said outlet which will suspend and withdraw through said outlet crystal particles which are below a predetermined size only, valve means in said vessel associated with each of said elutriation chamber outlets enabling any one or more of said elutriation chambers to be selectively openly communicated with said recycle stream for independently, with respect to said variable flow rate means, controlling the volume of slurry liquid and crystal particles removed from said elutriation chambers, and crystal particle solubilizing means in said apparatus for removing said crystal particles below said predetermined size in said recycle stream prior to the return thereof to said vessel.

7. An improved crystallization apparatus, comprising an enclosed vessel adapted to contain a slurry body of liquid and crystal particles, vacuum means communicating with said vessel for maintaining a supersaturation zone in said slurry body, said vessel including a mid-section defined by a generally cylindrical shell, a first interior wall co-axially disposed with respect to 7 said cylindrical shell and with said cylindrical shell defining a first elutriation chamber having a substantially uniform annular cross-section for at least a substantial portion of its height, a second interior wall-co-axially disposed with respect to said first interior wall, said first and second interior walls defining a second elutriation chamber having a substantially uniform cross-section for at least a substantial portion of its height, each of said elutriation chambers having an inlet adjacent the lower end thereof communicating with said substantial portion of the slurry body adjacent the periphery of said predetermined flow path and having an outlet above said inlet for withdrawing a recycle stream of slurry liquid and crystal particles, variable flow rate means communicating with each of said outlets for withdrawing the recycle stream therefrom at a rate which can be selectively controlled to establish a flow velocity in the elutriation chamber associated with said outlet which will suspend and withdraw through said outlet crystal particles which are below a predetermined size only, valve means in said vessel associated with each of said elutriation chamber outlets enabling any one or more of said elutriation chambers to be selectively openly communicated with said recycle stream for independently, with respect to said variable flow rate means, controlling the volume of slurry liquid and crystal particles removed from said elutriation chambers, and solubilizing means in said apparatus for removing said crystal particles below said predetermined size in said recycle stream prior to the return thereof to said vessel.

8. The improved crystallization apparatus of claim 7 wherein said baffle means includes a third interior wall co-axially dispoed with respect to said second interior wall, said second and third interior walls defining a third elutriation chamber also having a substantially uniform cross-section for at least a substantial portion of its height.

9. An improved crystallization apparatus comprising a vessel adapted to contain a slurry body of crystals and liquid, said vessel having a recycle inlet and outlet formed therein, a recycle loop communicating said recycle outlet with said recycle inlet, vacuum means communicating with said vessel for maintaining supersaturation conditions in at least a portion of the slurry body contained in said vessel, pump means in said recycle loop for providing forced circulation of said recycle slurry, a slurry removal outlet in said apparatus, a fines removal circuit communicating with said slurry removal outlet, said fines removal circuit including, at least two separate elutriation chambers, each of said elutriation chambers having an outlet at one end thereof for removing slurry liquid and crystal particles which fall below a predetermined size from a slurry stream withdrawn from the slurry body through said slurry removal outlet, means communicating with each of said elutriation chamber outlets to selectively oper ate any one or more of said elutriation chambers at a desired flow rate; whereby, the size and quantity of crystal particles removed from'said slurry body through said elutriation chamber outlets can be selectively and independently regulated with respect to each other to provide product crystals of improved size uniformity.

10. The improved crystallization apparatus of claim 9 wherein each of said elutriation chambers is contained in a settling tank wherein crystal particles above a predetermined size are removed through an outlet adjacent the bottom thereof which outlet is in direct communication with a return inlet in said apparatus.

11. The apparatus of claim 9 wherein said fines removal circuit includes a crystal particle solubilizing means wherein crystal particles below a predetermined size removed through said elutriation chamber outlets can besolubilized prior to return of the liquid slurry to the crystallization apparatus.

12. An improved crystallization apparatus comprising an enclosed vessel adapted to contain a slurry body of liquid and crystal particles, means in said vessel for maintaining supersaturation conditions in at least a portion of the slurry body contained therein, a recycle outlet formed in said vessel for withdrawing a portion of said slurry body as a recycle stream, said recycle outlet communicating with the suction side of a pump having a discharge side in communication with a recycle inlet formed in said vessel, said recycle inlet in said vessel communicating with associated recycle inlet piping adapted to discharge said recycle slurry into said vessel adjacent the central interior portion of said vessel, at least two elutriation chambers in said vessel, each of said elutriation chambers having an outlet at one end thereof communicating with a fines removal circuit, means communicating with each of said elutriation chamber outlets to selectively operate any one or more of said elutriation chambers at a desired flow rate; whereby, both the size and quantity of crystal particles removed from said slurry body via said elutriation chambers are independently regulated with respect to each other to provide product crystals of improved size uniformity.

13. An improved crystallization process, comprising a body of slurry which includes crystal particles and liquid in a vessel, maintaining conditions in said vessel for establishing supersaturation in said slurry body to induce crystallization therein, circulating said body of slurry in a predetermined flow path in said vessel at a flow rate sufficient to maintain said crystal particles in suspension, segregating a portion of said slurry body in a plurality of elutriation chambers, each of said elutriation chambers having a lower end in open communication with a main portion of said slurry body adjacent said predetermined flow path, withdrawing a stream of crystal particles and slurry liquid from each of said elutriation chambers through an outlet adjacent an upper end thereof and transmitting said stream to a fines removal circuit, each of said outlets including valve means enabling any one or more of said chambers to be in direct communication with said fines removal circuit, selectively controlling the velocity of slurry in said elutriation chambers to effect withdrawal of crystal particles below a predetermined size only in said stream, independently controlling the volume of slurry and crystal particles removed in said stream by selectively communicating any one or more of said elutriation chambers with said fines removal circuit, removing the crystal particles in said stream and thereafter returning said stream to said body of slurry in said vessel whereby, both the size and quantity of crystal particles removed from said slurry body in said stream are independently regulated to provide product crystals of improved size uniformity.

14. An improved crystallization apparatus comprising an enclosed vessel adapted to contain a slurry body of liquid and crystal particles, means in said vessel for maintaining supersaturation conditions in at least a portion of the slurry body contained therein, a recycle outlet formed in said vessel for withdrawing a portion of said slurry body at the recycle stream, said recycle outlet communicating with the suction side of a pump having a discharge side in communication with a recycle inlet formed in said vessel, said recycle inlet in said vessel communicating with associated recycle inlet piping adapted to discharge said recycle slurry in said vessel adjacent the central interior portion of said vessel, said recycle inlet piping including a generally vertically extending pipe portion having substantial length, an interior baffle coaxially disposed with respect to and around said pipe portion, said baffle and pipe portion defining a channel of substantially cross-sectional area for at least a substantial portion of its height. radially extending partitions between said pipe portion and baf fle defining said channel into at least two separate elu triation chambers, each of said elutriation chambers having an inlet adjacent the lower end thereof comm unicating with slurry contained in said vessel and an outlet at least a substantial height above said inlet, variable flow rate means communicating with each of said elutriation chamber outlets for selectively controlling the flow velocity in said elutriation chambers to establish a flow velocity therein for suspending only crystal particles below a predetermined size, and valve means operatively associated with each of said elutriation chamber below said predetermined size. 

1. AN IMPROVED CRYSTALLIZATION APPARATUS, COMPRISING A VESSEL ADAPTED TO CONTAIN A BODY OF SLURRY WHICH INCLUDES CRYSTAL PARTICLES AND LIQUID,MEANS FOR MAINTAINING SUPERSATURATION CONDITIONS IN THE SLURRY BODY TO INDUCE CRYSTALLIZATION THEREIN, MEANS IN SAID CRYSTALLIZATION APPARATUS FOR CIRCULATING SAID ALURRY IN A PREDETERMINED FLOW PATH IN SAID VESSEL AND FOR MAINTAINING SAID CRYSTAL PARTICLES IN SUSPENSION, SAID VESSEL ALSO INCLUDING AT LEAST TWO SEPARATE ELUTRIATION CHAMBERS, EACH OF SAID ELUTRIATION CHAMBERS HAVING AN OUTLET AT ONE END THEREOF FOR REMOVING SLURRY LIQUID AND CRYSTAL PARTICLES WHICH ARE BELOW A PREDETERMINED SIZE ONLY FROM SAID SLURRY BODY, AND MEANS COMMUNICATING WITH EACH OF SAID OUTLETS FOR INDEPENDENTLY REGULATING THE FLOW IN EACH OF SAID ELUTRIATION CHAMBERS TO SELECTIVELY OPERATE ANY ONE OR MORE OF SAID ELUTRIATION CHAMBERS AT A DESIRED FLOW RATE, WHEREBY, THE SIZE AND QUANTITY OF CRYSTAL PARTICLES REMOVED FROM SAID SLURRY BODY THROUGH SAID ELUTRIATION CHAMBERS CAN BE SELECTIVELY AND INDEPENDENTLY REGULATED WITH RESPECT TO EACH OTHER TO PROVIDE PRODUCT CRYSTALS OF IMPROVED SIZE UNIFORMITY.
 2. The improved crystallization apparatus of claim 1 wherein each of said elutriation chambers in sAid apparatus is of substantially uniform cross-section for at least a substantial portion of its height.
 3. An improved crystallization apparatus, comprising a vessel adapted to contain a body of slurry which includes crystal particles and liquid, means in said vessel for maintaining a supersaturation zone in said slurry body to induce crystallization therein, means in said vessel for circulating a substantial portion of said slurry body in a predetermined flow path through said supersaturation zone and for maintaining said crystal particles in suspension in said liquid, a plurality of elutriation chambers in said vessel, each of said elutriation chambers having a substantially uniform cross-section for at least a substantial portion of its height and a lower end which is in open communication with said substantial portion of said slurry body adjacent the predetermined flow path in a generally quiescent zone with respect thereto, each of said elutriation chambers also including an outlet adjacent the upper end thereof, means communicating with each of said outlets for independently regulating the flow rate in each of said elutriation chambers to withdraw a recycle stream of crystal particles and slurry liquid from said zone and to selectively operate any one or more of said elutriation chambers at a desired flow rate, thereby enabling the size and quantity of crystal particles removed from said slurry body to said recycle stream to be selectively and independently regulated with respect to each other, and means for removing crystal particles in said recycle stream prior to the return of said stream to the body of slurry in said vessel.
 4. An improved crystallization apparatus, comprising a vessel adapted to contain a body of slurry which includes crystal particles and liquid, first means for maintaining supersaturation conditions in the slurry body to induce crystallization therein, second means in said crystallization apparatus for circulating said slurry in a predetermined flow path in said vessel and for maintaining said crystal particles in suspension, a plurality of elutriation chambers in said apparatus, each of said elutriation chambers having a substantially uniform cross-section for at least a substantial portion of its height and having an inlet at one end thereof and an outlet at the opposite end thereof, said inlet communicating with said body of slurry at a location therein whereby slurry will be received in said elutriation chamber means from a generally quiescent zone with respect to said predetermined flow path, means communicating with each of said elutriation chambers for independently regulating the flow rate in each of said elutriation chambers to selectively operate any one or more of said elutriation chambers at a desired flow rate; whereby, both the size and quantity of crystal particles in said slurry stream removed from said slurry body can be selectively and independently varied with respect to each other to provide product crystals of improved size uniformity.
 5. The improved crystallization apparatus of claim 4 wherein said vessel includes two separate elutriation chambers, the first of which has a cross-sectional area which is equal to approximately one half the cross-sectional area of said second elutriation chamber.
 6. An improved crystallization apparatus, comprising an enclosed vessel adapted to contain a slurry body of liquid and crystal particles, vacuum means communicating with said vessel for maintaining a supersaturation zone in said slurry body, forced circulation means in said vessel for circulating a substantial portion of said slurry body in a predetermined flow path through said supersaturation zone and for maintaining said crystal particles in suspension, said vessel including a mid-section defined by a generally cylindrical shell and an interior wall co-axially disposed with respect to said cylindrical shell, said interior wall and cylindrical shell defining a channel having a uniform annular cross-section for at least a subStantial portion of its height, and partition members radially disposed between said interior wall and cylindrical shell defining said channel into separate elutriation chambers, each of said elutriation chambers having an inlet adjacent the lower end thereof communicating with said substantial portion of the slurry body adjacent the periphery of said predetermined flow path and having an outlet above said inlet for withdrawing a recycle stream of slurry liquid and crystal particles, variable flow rate means communicating with each of said outlets for withdrawing said recycle stream therefrom at a rate which can be selectively controlled to establish a flow velocity in the elutriation chamber associated with said outlet which will suspend and withdraw through said outlet crystal particles which are below a predetermined size only, valve means in said vessel associated with each of said elutriation chamber outlets enabling any one or more of said elutriation chambers to be selectively openly communicated with said recycle stream for independently, with respect to said variable flow rate means, controlling the volume of slurry liquid and crystal particles removed from said elutriation chambers, and crystal particle solubilizing means in said apparatus for removing said crystal particles below said predetermined size in said recycle stream prior to the return thereof to said vessel.
 7. An improved crystallization apparatus, comprising an enclosed vessel adapted to contain a slurry body of liquid and crystal particles, vacuum means communicating with said vessel for maintaining a supersaturation zone in said slurry body, said vessel including a mid-section defined by a generally cylindrical shell, a first interior wall co-axially disposed with respect to said cylindrical shell and with said cylindrical shell defining a first elutriation chamber having a substantially uniform annular cross-section for at least a substantial portion of its height, a second interior wall co-axially disposed with respect to said first interior wall, said first and second interior walls defining a second elutriation chamber having a substantially uniform cross-section for at least a substantial portion of its height, each of said elutriation chambers having an inlet adjacent the lower end thereof communicating with said substantial portion of the slurry body adjacent the periphery of said predetermined flow path and having an outlet above said inlet for withdrawing a recycle stream of slurry liquid and crystal particles, variable flow rate means communicating with each of said outlets for withdrawing the recycle stream therefrom at a rate which can be selectively controlled to establish a flow velocity in the elutriation chamber associated with said outlet which will suspend and withdraw through said outlet crystal particles which are below a predetermined size only, valve means in said vessel associated with each of said elutriation chamber outlets enabling any one or more of said elutriation chambers to be selectively openly communicated with said recycle stream for independently, with respect to said variable flow rate means, controlling the volume of slurry liquid and crystal particles removed from said elutriation chambers, and solubilizing means in said apparatus for removing said crystal particles below said predetermined size in said recycle stream prior to the return thereof to said vessel.
 8. The improved crystallization apparatus of claim 7 wherein said baffle means includes a third interior wall co-axially dispoed with respect to said second interior wall, said second and third interior walls defining a third elutriation chamber also having a substantially uniform cross-section for at least a substantial portion of its height.
 9. An improved crystallization apparatus comprising a vessel adapted to contain a slurry body of crystals and liquid, said vessel having a recycle inlet and outlet formed therein, a recycle loop communicating said recycle outlet with said recycle inlet, vacUum means communicating with said vessel for maintaining supersaturation conditions in at least a portion of the slurry body contained in said vessel, pump means in said recycle loop for providing forced circulation of said recycle slurry, a slurry removal outlet in said apparatus, a fines removal circuit communicating with said slurry removal outlet, said fines removal circuit including, at least two separate elutriation chambers, each of said elutriation chambers having an outlet at one end thereof for removing slurry liquid and crystal particles which fall below a predetermined size from a slurry stream withdrawn from the slurry body through said slurry removal outlet, means communicating with each of said elutriation chamber outlets to selectively operate any one or more of said elutriation chambers at a desired flow rate; whereby, the size and quantity of crystal particles removed from said slurry body through said elutriation chamber outlets can be selectively and independently regulated with respect to each other to provide product crystals of improved size uniformity.
 10. The improved crystallization apparatus of claim 9 wherein each of said elutriation chambers is contained in a settling tank wherein crystal particles above a predetermined size are removed through an outlet adjacent the bottom thereof which outlet is in direct communication with a return inlet in said apparatus.
 11. The apparatus of claim 9 wherein said fines removal circuit includes a crystal particle solubilizing means wherein crystal particles below a predetermined size removed through said elutriation chamber outlets can be solubilized prior to return of the liquid slurry to the crystallization apparatus.
 12. An improved crystallization apparatus comprising an enclosed vessel adapted to contain a slurry body of liquid and crystal particles, means in said vessel for maintaining supersaturation conditions in at least a portion of the slurry body contained therein, a recycle outlet formed in said vessel for withdrawing a portion of said slurry body as a recycle stream, said recycle outlet communicating with the suction side of a pump having a discharge side in communication with a recycle inlet formed in said vessel, said recycle inlet in said vessel communicating with associated recycle inlet piping adapted to discharge said recycle slurry into said vessel adjacent the central interior portion of said vessel, at least two elutriation chambers in said vessel, each of said elutriation chambers having an outlet at one end thereof communicating with a fines removal circuit, means communicating with each of said elutriation chamber outlets to selectively operate any one or more of said elutriation chambers at a desired flow rate; whereby, both the size and quantity of crystal particles removed from said slurry body via said elutriation chambers are independently regulated with respect to each other to provide product crystals of improved size uniformity.
 13. An improved crystallization process, comprising a body of slurry which includes crystal particles and liquid in a vessel, maintaining conditions in said vessel for establishing supersaturation in said slurry body to induce crystallization therein, circulating said body of slurry in a predetermined flow path in said vessel at a flow rate sufficient to maintain said crystal particles in suspension, segregating a portion of said slurry body in a plurality of elutriation chambers, each of said elutriation chambers having a lower end in open communication with a main portion of said slurry body adjacent said predetermined flow path, withdrawing a stream of crystal particles and slurry liquid from each of said elutriation chambers through an outlet adjacent an upper end thereof and transmitting said stream to a fines removal circuit, each of said outlets including valve means enabling any one or more of said chambers to be in direct communication with said fines removal circuit, selectively controlling the velocity of slurry in said elutriation chamBers to effect withdrawal of crystal particles below a predetermined size only in said stream, independently controlling the volume of slurry and crystal particles removed in said stream by selectively communicating any one or more of said elutriation chambers with said fines removal circuit, removing the crystal particles in said stream and thereafter returning said stream to said body of slurry in said vessel whereby, both the size and quantity of crystal particles removed from said slurry body in said stream are independently regulated to provide product crystals of improved size uniformity.
 14. An improved crystallization apparatus comprising an enclosed vessel adapted to contain a slurry body of liquid and crystal particles, means in said vessel for maintaining supersaturation conditions in at least a portion of the slurry body contained therein, a recycle outlet formed in said vessel for withdrawing a portion of said slurry body at the recycle stream, said recycle outlet communicating with the suction side of a pump having a discharge side in communication with a recycle inlet formed in said vessel, said recycle inlet in said vessel communicating with associated recycle inlet piping adapted to discharge said recycle slurry in said vessel adjacent the central interior portion of said vessel, said recycle inlet piping including a generally vertically extending pipe portion having substantial length, an interior baffle coaxially disposed with respect to and around said pipe portion, said baffle and pipe portion defining a channel of substantially cross-sectional area for at least a substantial portion of its height, radially extending partitions between said pipe portion and baffle defining said channel into at least two separate elutriation chambers, each of said elutriation chambers having an inlet adjacent the lower end thereof communicating with slurry contained in said vessel and an outlet at least a substantial height above said inlet, variable flow rate means communicating with each of said elutriation chamber outlets for selectively controlling the flow velocity in said elutriation chambers to establish a flow velocity therein for suspending only crystal particles below a predetermined size, and valve means operatively associated with each of said elutriation chamber outlets for selectively operating any one or more of said elutriation chambers simultaneously, whereby both the size and quantity of crystal particles below a predetermined size can be independently regulated to control the residence time in said vessel of said crystal particles below said predetermined size. 